As technology development and demand for a mobile device have increased, demand for a secondary battery as an energy source has rapidly increased. Conventionally, a nickel-cadmium battery or a hydrogen ion battery has been used as the secondary battery. However, a lithium secondary battery is recently widely used because charging and discharging is free due to rare memory effect in comparison with a nickel-based secondary battery, a self-discharge rate is very low, and an energy density is high.
The lithium secondary battery mainly uses a lithium oxide and a carbonaceous material as a positive electrode active material and a negative electrode active material, respectively. The lithium secondary battery includes an electrode assembly in which a positive electrode plate and a negative electrode plate, respectively coated with the positive electrode active material and the negative electrode active material, are arranged with a separator therebetween, and an outer member, that is a battery case, which seals and receives the electrode assembly together with an electrolyte solution.
The lithium secondary battery includes a positive electrode, a negative electrode, and a separator and an electrolyte interposed therebetween. Depending on which material is used for the positive electrode active material and the negative electrode active material, the lithium secondary battery is classified into a lithium ion battery (LIB) and a polymer lithium ion battery (PLIB). Generally, an electrode of the lithium secondary battery may be prepared by applying the positive or negative electrode active material to a current collector made of aluminum or copper sheet, mesh, film, foil, or the like and then drying the same.
FIG. 1 is a perspective view showing a conventional battery module in which battery cells are stacked, and FIG. 2 is a cross-sectional view, taken along the line A-A′ of FIG. 1. Referring to FIGS. 1 and 2, a conventional battery module 1 in which battery cells 3 are stacked is configured so that a plurality of battery cells 3 are stacked on each other and accommodated in a case 2. Thus, the battery module 1 has an increased height H, and thus it is difficult to dispose the battery module 1 at an under floor of an electric vehicle or the like.